1. Field of the Invention
The invention relates in general to a digital television broadcasting technique, and more particularly, to a technique for determining a carrier frequency offset of a digital television signal.
2. Description of the Related Art
With advancements in communication techniques, digital television broadcasting gradually matures. Apart from being transmitted via cables, digital television signals can also be transmitted in a form of wireless signals via a base station or a satellite. The Digital Video Broadcasting—Satellite (DVB-S) and the Digital Video Broadcasting—Satellite—Second Generation (DVB-S2) are prevalent standards in digital television broadcasting.
In DVB-S and DVB-S2 specifications, symbol rates of digital television signals range between 0 to 45 MHz, while carrier frequencies of digital television signals range between 950 to 2150 MHz. As a result, the possible combinations of the two values above are rather formidable. Since a receiver cannot in advance be informed of a carrier channel and a symbol rate selected by the transmitter, it is critical that the receiver must be capable of accurately determining the two values in order to decode and restore a received signal.
In one conventional method for determining a carrier frequency offset, fast Fourier transform (FFT) is performed on an input signal, and a spectral line is generated by superimposing energy square values of all transform results, as shown in FIG. 1A. In the spectral line, a difference between a center C1 of a high-energy region and a center R (corresponding to one-half of computing points in the foregoing FFT) of the horizontal coordinate corresponds to a size of the carrier frequency offset.
As being interfered by in-channel noises or signals of neighboring channels during wireless transmission, not all signal spectrums appear as ideal as that shown in FIG. 1A. For example, an actual spectrum of an input signal may have two energy peaks as shown in FIG. 1B. A possible reason causing such spectrum is that the noise interference between the energy peaks is particularly severe. Another possible reason is that a neighboring channel is too close to a frequency band of desired target signal. If the occurrence of one of the energy peaks is contributed by the neighboring channel, a carrier frequency offset directly determined according to the spectrum inevitably contains an error. Yet, a current digital television broadcasting receiver system falls short in effectively determining which of the energy peaks truly represent(s) a desired target signal.